Now, I know that several Members built this circuit with red LEDs and reported correct results, but I still don't know why it was different in my case.

Regards,
Braca

The version on my webpage uses 3 diodes instead of an LED, it is a bit old. Since the current boards use an LED, they seem to have a bit more voltage drop. You could replace the LED with 3 1N4148 in series, although the IR LED is apparently also a solution.

I'm about to build another two K-Multipliers, and have now a few more IR LEDs at my disposal, so the table from post #1130 will probably get an update.
However, in my experiments with the PCB offered by Keanu, only IR LEDs with shorter wavelengths (< 900nm) seem to have been capable of producing the target voltage drop of 1.8V across Q2.

As promised, here are the voltage drop measurements for several LEDs in my K-Multiplier with BC334-40/Toshiba TTC004B (the circuit diagram from this thread, assembled on the Kean's PCB):It seems that only the 850nm (wavelength) IR LED produces the desired voltage drop of 1.8V across the pass transistor Q2.
The results for the negative polarity K-Multiplier (BC327-40/Toshiba TTA004B) are virtually the same, so I'm not quoting them.